swift234

fix rdar://25271859 QoI: Misleading error message when expression result can't be inferred from closure

We previously produced the error message:

rdar25271859.swift:14:11: error: value of tuple type '(Float, Int)' has no member '0'
  a.map { $0.0 }
          ^~ ~

We now produce:

rdar25271859.swift:15:5: error: generic parameter 'U' could not be inferred
   .andThen { dataResult in
    ^

which is at least is correct, if not yet helpful.